Contents

INTRODUCTION

Sea ice covers some 3-7% of the total surface of our planet depending on the season of the year [1]. Apart from being one of the most important climatic variables and key indicator of climate change, sea ice also provides an extreme and changeable habitat for diverse sympagic organisms, which play an important role in the ecosystems of the polar seas [2].

SEA ICE

Sea ice in the Arctic. Fot. Wojtek Moskal

Sea ice cover occurs primarily in the polar regions, but in the northern hemisphere it may be also found at lower latitudes (eg. in the Baltic, Caspian and Okhotsk seas, the Gulf of St. Lawrence, Scandinavian fjords) [3]. Land-fast ice forms and remains fast along the coast, attached to the shore or grounded to a shallow sea bottom. Pack ice refers to any area of floating sea ice that is not land-fast.

Sea ice formation is a relatively slow process as it demands cooling the whole upper layer of water column to the freezing temperature. Unlike fresh water, sea water attains its maximal density at its freezing point. Typical ocean water freezes only at about -1.8°C. As the ocean temperature nears the freezing point, the water density increases and the water sinks. Due to stratification of the water column, thermohaline convection involves only the surface layer down to the pycnocline level. Usually the top 100 to 150 meters of water must be cooled to the freezing temperature for ice to form. Sea ice formation begins with small (ca 3-4 mm in diameter) ice crystals called frazil, that accumulate and bond together as freezing process continues. During this process most of the salt is expelled into the water. Some brine, however, become trapped in small spaces between ice crystals and remains in a liquid state. Therefore, at the initial stages of its formation sea ice has a relatively high salt content. Over time brine drains out and the salinity of sea ice decreases.

Different types of ice cover may develop depending upon local hydrodynamic conditions. In calm waters congelation growth of ice with columnar texture results in forming ice sheets with smooth bottom surface, while in rough waters frazil growth dominates and sheet ice is formed from consolidated pancakes with a rough, jagged bottom. Once sea ice forms into sheet ice, it continues to grow through the winter. First-year ice is a sea ice of not more than one winter’s growth. Ice which has survived one or more summer seasons of partial melt is called multi-year ice.

Sea ice is dynamic. Its thickness, structure and condition are shaped by the interplay of different processes. Ice cover undergoes the annual cycle of summer melting and winter accretion. Sea ice, except fast ice, is almost continually in motion, driven mainly by the wind and sea currents. Collisions of ice masses create ridges and keels, which can considerably enlarge the ice thickness. Open water areas occur within sea cover as leads and polynyas opens and shift. Ice surface’s properties may be modify by presence or absence of snow cover and melt ponds [4].

Arctic vs Antarctic

Sea ice differs between the Arctic and Antarctic, primarily because of their different geography (Table 1).
The Arctic Ocean is a semi-enclosed basin with shallow marginal seas surrounded by land, with limited water and ice exchange with the North Atlantic. Most of existing ice remains within the Arctic Basin, where it is subjected to change driven by melting and freezing cycle and deformation through rafting and ridging of floes. More than half of the sea’s ice in the Arctic is multi-year ice typically ranging from 2 to 4 meters thick. Both, the extent and thickness of the Arctic perennial multi-year ice has been significantly decreasing over past thirty years.
In contrast, in the southern hemisphere ice forms around the edges of vast frozen continent, surrounded by the deep Southern Ocean, which is circumpolar and unbounded at its northern extent. Majority of the Antarctic ice is first-year ice less than 1m thick and melts in summer. Sea ice extent in the southern hemisphere has shown an overall increasing trend [2][3].

Microbiology of the sea ice

The incorporation of micro-organisms from the water column into the sea ice occurs mainly during the ice formation. Ice crystals floating on water surface act as filter collecting particles such as micro-algae, heterotrophic protists and bacteria. Subsequent changes in species composition occurs due to selection, succession and later colonization [5]. Major primary producers in the ice are pennate diatoms and flagellated protists [6][7]. The growth of ice algae is limited mainly by light and nutrient availability, whereas space within ice (brine volume) may determine vertical and seasonal variations in species distribution [8]. Annual production of sea ice autotrophs ranges from 5 to 15 gCm-2yr-1 in the Arctic and from 0.3 to 34 gCm-2yr-1 in the Antarctic [9]. Ice algae contribute considerably to the total primary production in the Arctic (25%) and in the Antarctic (20%) and play an important role in sustaining the secondary production that supports marine mammals and birds [10]. Heterotrophic micro-organisms in sea ice include several groups of protists as well as fungi, bacteria and archaea [5].

Macrobiology of the sea ice

Metazoans can use sea ice either as temporary breeding, nursery and feeding ground or refuge area (allochthonous) or as a permanent habitat during the entire life cycle (autochthonous) [11].
The brine–channel system within the sea-ice matrix is inhabited by small-bodied organisms dominated by rotifers, nematodes and turbellarians in the Arctic and harpacticoid copepods and turbellarians in the Antarctic. They tend to concentrate in the bottom ice layers which is more favourable microhabitat than the surface layer due to less stressful temperature and salinity conditions, abundance of algal food and complex ice structure. This community may be seasonally enriched by larvae of polychaetes and molluscs in the shallow waters.

Sympagic amphipod Onisimus nanseni Fot. Sławek Kwaśniewski

The ice underside constitute an important habitat for many polar organisms, especially large crustacean grazers. Species composition, distribution and abundances are highly related to age, type and morphology of ice. Gammaridean amphipods (Gammarus wilkitzkii, Apherusa glacialis, Onisimus nanseni and O. glacialis) occupy ice-water interface throughout the Arctic. Euphausiids species (Euphausia superba and E. crystallorophias)dominate underneath Antarctic sea ice [12]. Interspecific competition between crustacean grazers is reduced due to discrete differences in their trophic strategies and spatial segregation [13][14].

In both polar regions high concentration of calanoid copepods along with many other pelagic organisms such as pteropod gastropods, siphonophores, appendicularians, chaetognaths, hyperiid amphipods, mysids and benthic invertebrate larvae can be found in the water column directly under the ice [12]. Under-ice fauna is an important food source for the apex predators such as fish, birds and mammals. The polar cod (Boreogadus saida) feeding on copepods and amphipods is a key species between lower and upper trophic levels in Arctic food web as it represents the important food item for marine mammals and birds [15]. A broadhead fish (Pagothenia borchgrevinki) exerts significant predation pressure on the variety of ice-associated crustaceans in the Antarctic ice-covered regions [12].

Ice-associated mammals and birds

Many birds and mammals are strongly associated with ice which serves as a platform for resting, feeding and reproduction and provides a refuge from predators. The survival of the animal populations is highly related to the presence of areas of ice-free waters within sea ice cover which provide migration routes and enables access to abundant under-ice food resources. Polynyas which remain open throughout the winter or open at the same place each spring are areas of special ecological significance to overwintering or migrating species such as walruses, certain seals, bowhead whales, narwhals and belugas. Different habitat preferences and adaptations have evolved among ice-associated animals. The fast ice in the Arctic is occupied by ringed seals which require sufficient snow cover on the ice to construct birth lairs and the ice stability for the successful rearing of their pups. Bearded seals and walruses prefer regions of thin or broken ice cover over relatively shallow depths since their main food source are benthic invertebrates. Highly migratory, pelagic species such as hooded and harp seals give birth to their pups on heavy ice floes within ice-edge zones. Polar bears usually rely on fast ice to search for ringed and bearded seals, but they also hunt for other seal species, walruses and belugas along leads systems within pack ice. In the Antarctic Weddell seals and emperor penguins characterize the fast ice zone, while crabeater and leopard seals are species of deep pack ice. The distribution and migration patterns of polar cetaceans are strongly influenced by ice cover and feeding opportunities. They primarily occur in highly productive ice marginal zones feeding on dense populations of crustaceans, fish, and other animals such as squids. Killer whales are the apex predators hunting for an diverse array of species including penguins, seals and other cetaceans [16].

THREATS

Polar bears which depend on sea ice for many aspects of their life history are particularly vulnerable to the effects of habitat loss. Fot. Wojtek Walkusz

Polar regions have experienced significant warming in recent decades. Climate-inducted changes have been most pronounced across the Arctic Basin and along the Antarctic Peninsula, with significant decreases in the extent, thickness and seasonal duration of sea ice. These changes have severe ecological consequences for the sea ice biota. The habitat loss and changes in productivity, species composition and community structure of the under-ice community have a negative impact on higher trophic levels. Warming can also affect the sea ice ecosystem through changes in hydrography which include introduction of species from lower latitudes. The decline of the sea-ice extent will improve the accessibility of the high latitude areas. That may led to increase of anthropogenic pressure on polar ecosystems (ship traffic, exploration, industrial activities and fisheries) [16][17][18].